The ocean weighs a lot. Probably. We're pretty sure it does, at any rate. Figuring out precisely how much the ocean weighs will help us to model sea level rise, and so some oceanographers want to go drop a scale into the Pacific and see what it says.
The ocean fills a bit and empties a bit every year in a seasonal cycle. During the Northern summer, between March and September, something like six million tons of "extra" water pours into the ocean from melting land ice, which is enough to raise global sea levels by nearly two centimeters. Over the winter months, almost all (but not all) of this water will evaporate back out of the ocean and get rained or snowed back down over the ground.
It's important to be able to figure out just how much of this seasonal water doesn't make it back out of the ocean in order to more accurately model how much sea levels are rising from year to year, and in order to do that, we have to be able to keep track of the weight of the ocean. Volume is the other piece of the puzzle, but while the volume of the ocean changes with temperature, the weight is an actual measure of how much water there really is.
The way to go about doing this, according to researchers at the National Oceanography Centre in jolly old England, is basically to just go out into the middle of the Pacific and drop a scale overboard. And that's it. The researchers say that "making accurate measurements of changing pressure at a single point will indicate the mass of the world ocean." Seems easy enough, as soon as someone figures out how to build a scale that can measure the pressure of fractions of a millimeter of water while under four kilometers or so of the stuff, which amounts to about one part in 10 million.